Systems for communicating between a transponder in an electronic implanted device and a two-way telemetry device placed in close proximity to the implanted device are known in the art. For example, U.S. Pat. No. 4,361,153 discloses an external unit which transmits a carrier signal. The implanted device includes a transponder having a tuned resonant circuit for resonating at the frequency of the carrier signal so as to re-radiate a signal at the carrier frequency. The transponder also includes means for superimposing an information signal on the re-radiated signal by altering the resonance in accordance with the information signal. The re-radiated signal is received by the external device and the information signal is extracted therefrom. The resonant circuit in this patent consumes only the energy required to modulate the transponder's load impedance. Thus, the energy consumption level of this circuit is very low compared to the energy of the total signal reflected by the transponder.
Other telemetry systems for implanted devices are described in U.S. Pat. Nos. 3,576,554, 4,160,971, 4,281,664, 4,361,153, 4,494,545, 4,571,589, 4,854,328, 4,944,299, 4,952,928, 5,250,944 and 5,252,962.
Low power passive radio frequency (RF) identification transponder/reader systems are also known in the art. One system is described in Page, Raymond, "A Low Power RF ID Transponder," RF Design, July 1983, 31, 32, 34, 36. In this system, the reader transmits an unmodulated RF interrogation signal to a transponder attached to an object such as a rail car. The transponder receives the unmodulated RF signal, and returns a frequency doubled (i.e., second harmonic) AM modulated RF signal to a receiver placed a short distance away from the transponder. The frequency doubling and modulation are performed in the transponder by a single microwave diode. The return signal is modulated with information identifying the object. For this system to work, the reader and transponder must be in close proximity to each other.
Despite the substantial efforts in the prior art to build telemetry systems for communicating between a low-power, passive transponder implanted in an object and an interrogating device external to the object, there are still shortcomings in the existing systems. One significant shortcoming is that the interrogating device must be in close proximity to the transponder, thereby limiting the range of the telemetry system. Thus, truly "remote" monitoring of the implanted object is not possible. Another shortcoming in many prior art passive transponder systems is that the transponders rely upon the interrogating signal to gain the energy for transmission and modulation. Such systems require relatively high power interrogating signals and rectifying circuitry in the transponder. Another shortcoming in prior art telemetry systems employing passive transponders is that the antennas in the systems, and especially in the transponders, have high levels of interference and clutter, and do not function well with extremely low power signals. Yet another significant shortcoming of prior art active telemetry systems for implanted devices is that their maximum data rates (i.e., bit rates) are limited due to the use of a low carrier frequency. These telemetry systems typically employ coil-type antennas which cannot transmit or receive high carrier frequencies. Typical data rates are about 1 to 2 kilobits per second (1 to 2 kbps), with a maximum rate of about 8 kbps.
Thus, there is still a need for a telemetry system which can interrogate an implanted object from a distant monitoring station, which employs a passive-type transponder in the implanted object, which employs an extremely low power transponder that does not require a high power interrogating signal or rectifying circuitry for operation, which employs an antenna design that performs well with extremely low power signals and which can operate at high data rates. The present invention fills that need.